You will be prompted to make
You will be asked for the period which is the same for all periodic variables (if they must be different, rescale them)
Choose them by name or by clicking the appropriate names from the list presented to you. An X will appear next to the selected ones. Clicking toggles the X .
Type Esc when done or CANCEL or DONE.
XPP mods your variables by this period and is smart enough when plotting to not join the two ends.